An electromagnetic actuator configuration at which the present invention is directed is described in International Publication No. WO2004/097184 (filed by the present applicant), the contents of which are incorporated herein by reference. The present invention seeks to provide improvements to this actuator arrangement.
An actuator configuration described in this publication is shown in present FIGS. 1A and 1B. They show front and rear perspective views respectively. A rotor 10 is rotatably mounted in a housing 12 for rotation about an axis 14. It is surrounded by a stator 16 comprising eight poles. A respective winding 18 is wound around each pole.
A lever 20 is pushed on to the cam surface 24 of a cam 22 by a leaf spring 26. Cam surface 24 is cylindrical and eccentrically mounted on the rotor with respect to the rotor's axis 14. The actuator is coupled to a valve stem 30. It is arranged such that maximum deflection of the leaf spring 26 occurs when the valve stem 30 is at the upper end of its vertical travel, that is, in the valve closed position.
The coupling between the actuator and valve stem 30 is visible in FIG. 1B. A crank pin 40 extends from the rear of the rotor, through a lever 42. Lever 42 is mounted so as to be pivotal about an axis 44. The crank pin 40 passes through an aperture defined by lever 42, the wall of which defines a cam surface 46. This follows the movement of the crank pin as it rotates, converting this rotational movement into substantially vertical oscillation of the valve stem 30 via pivotable coupling 48, providing desmodromic valve control.
The passive magnetic forces between the rotor and stator serve to define eight stable rest positions for the rotor. In each rest position, the rotor is firmly held in position by these passive magnetic forces without requiring the input of energy (such as an electric current though the stator windings).
The rotor can be rotated from one rest position to another by applying a suitable current pulse to one or more stator windings. The eight windings (or coils) are connected together in four pairs, with each pair consisting of two windings on opposite sides of the rotational axis 14. The windings in each pair may be connected together in series or in parallel.
The actuator is controllable to energise one pair, or two pairs, or all four pairs of windings depending on the magnitude of the required impulse. This can vary substantially depending on a range of factors such as engine speed, valve stiffness, oil viscosity, and temperature, for example.
The leaf spring stores energy as the valve stem moves into its closed position. This energy is then used to accelerate the rotor when it moves away from this primary rest position by virtue of the action of the spring 26 on the rotor via lever 20 and cam 22. This may substantially reduce the peak electric current required to shift the rotor in the direction away from its rest position. As noted above, rotation of the rotor is converted into movement of the valve stem via the linkage shown in FIG. 1B.
As the valve stem moves back towards its closed position, the leaf spring serves to control and reduce its speed as it approaches its seat. This helps to reduce engine noise and increase the life of the engine. At the same time, kinetic energy is stored in the spring for reuse during the valve opening phase.